Fix tests where the compiler fails to infer types.

At least as of 6.1b3, Swift won’t infer `CInt` for the integer literals passed to `contain()`. I opted to cast, but it may be that the test isn’t actually relevant given this failure.

It may also be that this should be filed as a bug against Swift.
An alternative might be to remove the `NMBCollection` override and add a `CollectionType` override relying on `countElements()`: `CollectionType` is more specific than `SequenceType` so it won’t run into this ambiguity.